Origin of higher graphitization under higher humidity on the frictional surface of self-mated hydrogenated carbon films

Abstract

The ‘friction-induced graphitization’ mechanism for hydrogenated amorphous carbon (a-C:H) films has been postulated, but it requires concrete analyses at specific conditions. Here, we have discussed the reason for higher sp2 bond content on the interface of self-mated fullerene-like a-C:H films when the relative humidity (RH) increases from dry N2 to 70%. The reason is the failure of the role of FL structures and the change of interface structure evolution. Before RH 50%, FL structures work properly, and the films remain low graphitization to form nanoparticle-like debris having onion-like structures. After RH 50%, the failure of the role of FL structures leads to the abruptly increasing wear, and the formation of larger flake-like debris. Higher sp2 content, is not only caused by formation of longer and order graphene sheets than the original surfaces, but also linked with the increase of CO bonds. This is unlike the general friction-induced graphitization that higher sp2 content indicates higher graphitization. The result reminds us to further understand friction-induced graphitization mechanism at special condition.